EP-A-1 095 634 (EP 634) discloses a rapid-exchange, self-expanding stent delivery system. EP 634 discloses a system in which the soft atraumatic distal tip of the system is at the leading end of the inner catheter. The outer sheath of the delivery system has a distal end which stops proximally short of the atraumatic tip.
Stents to be deployed at a stenting site within a human or animal body expand radially in the course of delivery, from a radially compact delivery disposition to a radially larger deployed disposition. In self-expanding stents made of stainless steel, the deformation of the stent is below the elastic limit, the stent until its deployment being radially confined and under elastic stress and typically released by proximal withdrawal of a confining sheath while the stent is itself prevented from moving proximally with the confining sheath by abutment with a stop on the distal end of a catheter shaft which suffers axial compressive stress while the surrounding sheath is proximally withdrawn.
By contrast, stainless steel stents which are relaxed in a radially compact disposition suffer plastic deformation when expanded into their deployed disposition by inflation of a balloon within the lumen of the stent.
An early example of stainless steel self-expanding stents is Gianturco U.S. Pat. No. 4,580,568 and an early example of the balloon expansible stainless stent is Palmaz EP-A-221 570.
A third category of stent is the memory metal stent, made of a biologically compatible nickel-titanium shape memory alloy with martensitic and austenitic phases. At body temperature, the stent “seeks” to revert to the austenitic phase. Typically it is confined within a surrounding sheath and again released at the stenting site by proximal withdrawal of this sheath.
The present invention offers improvements in systems to deliver those stents which are brought to the stenting site within a confining surrounding sheath.
In the technical field of stenting, there is a desire to reduce the transverse dimensions of the stent delivery system. In this field, the widely used measure of transverse cross-section is the unit of “French”, often abbreviated to “F” which is a one third part of a millimeter. Thus, a 6 French (6 F) delivery system has a diameter of 2 millimeters.
For any particular stenting operation, one has to select a particular stent and a particular delivery system. There is a large choice in both of these elements. Accordingly, it would be an advantage for manufacturers of stents and their delivery systems to achieve a degree of modularity in the design and construction of stents and their delivery systems. For example, there is a wide range of stents which could be delivered by a 6 F delivery system and it would therefore be convenient for the manufacturer of a stent delivery system to be able to tailor a basic 6 F system to fit any particular stent which would be compatible with a 6 F delivery system. This would reduce costs, to the advantage of patients, while retaining full flexibility for medical practitioners to optimise their choice of stent for any particular patient.
Like many catheter systems and trans-luminal surgical devices, a stent delivery system is often used with a flexible guidewire. The guidewire is preferably made of metal, and is slidably inserted along the desired body passage. The delivery system is then advanced over the thus pre-placed guidewire by “backloading” or inserting the proximal end of the guidewire into a distal guidewire port leading to a guidewire lumen defined by the delivery system.
Many conventional trans-luminal surgical devices delivery systems define guidewire lumens that extend along the entire length of the outer sheath. These delivery systems are described as “over-the-wire” delivery systems, in that the surgical device is guided to the site of the surgery over the guidewire, the guidewire thereby exiting the delivery system at the proximal end of the delivery system. “Over-the-wire” delivery systems provide several advantages, including improved trackability, the ability to flush the guidewire lumen while the delivery system is inside the patient's body, and easy removal and exchange of the guidewire while the delivery system remains in a desired position in the patient.
In some circumstances, however, it may be desirable to provide a “rapid exchange” delivery system, which offers the ability more easily to remove and exchange the delivery system while retaining the guidewire in a desired position within the patient. In a rapid-exchange delivery system, the guidewire occupies a lumen located only in the distal portion of the delivery system. The guidewire exits the delivery system through a proximal guidewire port, closer to the distal end of the delivery system than to its proximal end, and extends in parallel along the outside of the proximal portion of the delivery system.
Because a substantial length of the guidewire is outside the delivery system, it may be manually held in place close to the point where it passes the entry point on the body of the patient, as the delivery system is removed. This facilitates handling, removal and exchange of the delivery system for the practitioner for the following reasons.
With a guidewire lumen that is much shorter than the full catheter length a single physician can insert and remove a stent (or other surgical device) delivery system into and from the patient's body. Whereas conventional delivery systems require a guidewire having a length at least double the length of the outer catheter, the rapid-exchange configuration allows the use of much shorter guidewires which enable a single physician to handle the proximal end of the guidewire at the same time as the catheter at the point of its entry into the body of the patient.
Accordingly, the present invention advantageously provides a stent delivery system having a rapid-exchange configuration for delivering and deploying a self-expanding stent or other trans-luminal surgical element, or performing a surgical procedure in a percutaneous, trans-luminal manner.
Stents themselves cannot be directly seen by the naked eye during a trans-luminal journey to the stenting site, nor can one directly see whether the stent is exactly located as desired within the stenting site. Rather, indirect means have to be used to follow the progress of the stent through the body and make sure that it is correctly located before it is deployed. Thus, the device delivery system is used during deployment to carry radiopaque contrast or marker fluid to the site of surgery so that the target site can be seen through the radiopaque fluid in the bodily lumen at the site. This radiopaque fluid is generally injected through an injection port at the proximal end of the delivery system and through an annular space between an outer sheath of the delivery system and a proximal portion of an inner catheter shaft. The visibility of the site is adversely affected when the lumen, through which radiopaque contrast fluid is injected, is too small at the site to deliver a strong pulse of contrast fluid. As pulses of fluid are used for visualisation, the effectiveness of visualisation depends on the volume flow in each pulse. This in turn depends on the ease of flow of the fluid along the full length of the delivery system, from the point of injection at the proximal end, to the site of surgery beyond the distal end of the delivery system.
Thus, delivery systems which offer a large cross-section and unimpeded lumen for contrast fluid will be favoured by radiologists, other things being equal. The visibility can additionally be increased by further reducing the resistance of the system to pulses of contrast fluid. It is therefore an object of the present invention to provide good visualisation with contrast fluid, without sacrifice of other important performance aspects of the delivery system, including pushability and low overall diameter. By increasing “pushability” we mean the capability to be advanced longer distances along narrower and more tortuous bodily lumens.
Furthermore, the delivery system invariably carries at least one radiopaque marker at a known location relative to the length of the surgical device (such as a stent), so that radiologists can be sure of the location of the ends of the device, on the basis of their knowledge of the location of the radiopaque marker. Even if the device is rendered sufficiently radiopaque for it to be seen, it is still useful to have a radiopaque marker on the distal end of the delivery system, to reveal for example successful separation of the device from the delivery system.
Thus, in our example of a 6 F delivery system, to be used for delivering stents of various lengths, there will be a wish to provide radiopaque markers within the delivery system at two spaced-apart locations on the axis of the delivery system, corresponding to the opposite ends of the stent (until the stent is deployed out of the system). One object of the present invention is to offer a degree of modularity in this design aspect.
With delivery systems having a rapid-exchange configuration, just as with over-the-wire systems, the delivery system is advanced over the guidewire, itself normally within a guide catheter, in order to bring the distal tip and surgical device to the site of surgery. Depending on the application, different diameter guidewires are specified. Two commonly used guidewire diameters are 0.46 mm/0.018 inches and 0.89 mm/0.035 inches (commonly known as 18 thou or 35 thou guidewires). Thus, a further degree of modularity can be achieved by offering a delivery system which is compatible with a range of guidewire diameters, specifically, both 18 thou and 35 thou guidewires.
Naturally, it would be an advantage for any new stent delivery system to be able straightforwardly to take the place of those previous delivery systems which individual practitioners have grown to be comfortable using. One such system uses in its proximal shaft portion a metal “hypo” tube, which can be made of stainless steel or another biocompatible alloy such as the cobalt/chromium/nickel alloy known by the trademark PHYNOX. The tube usually contains a push rod and the tube and rod move axially relative to each other to release the stent.
Further, it goes almost without saying, that good design for delivery systems for surgical devices such as stents is indicated by manufacturing steps which can be performed with high precision and reliability, yet with acceptable cost levels. This is yet another objective of the present invention.
Finally, for any system which is extremely long in proportion to its diameter, and features at least three co-axial elements, the cylindrical surfaces of these co-axial elements need to be so composed and conformed that friction between them is low enough that the co-axial elements can be moved tolerably easily axially relative to each other. It is yet another object of the present invention to provide systems which offer possibilities for bringing these friction levels down to advantageously low levels.
Along with all these issues already appreciated by those skilled in the art, there is a further performance aspect which becomes evident when a self-expanding stent is released progressively by successive proximal stepwise movements of the outer confining sheath.
Typically, the delivery system is extremely long in proportion to its cross-sectional dimensions, and is constructed predominantly or wholly from synthetic polymeric materials which have substantial elasticity and marked kinetic aspects to their deformation characteristics. In such a case, any particular rate of strain imposed on the proximal end of the outer sheath is likely to be experienced at the distal end of the same sheath in a somewhat different strain rate. For example, a fast squeeze of the trigger of a deployment system at the proximal end of the sheath will likely result in a somewhat slower resulting proximal advancement of the distal end of the same sheath. Furthermore, a pull on the sheath will impose compressive stresses along the length of the inner shaft, likely leading to a proximal movement of the stent which then relaxes back to the original, more distal, position of the stent as the tensile stress in the outer sheath eases back towards zero. In its own delivery systems, Applicant has observed what happens at the distal end of a stent delivery system during successive squeezes of the trigger of a delivery system which pulls the outer sheath proximally in a series of steps. The appearance at the stent end of the system is as if the system were “breathing” in that it, and the stent, moves axially first proximally, then distally, with each squeeze of the trigger.
This “breathing” phenomenon is of course a complicating factor when it comes to precision of intra-luminal placement of a stent (or other surgical element) within any particular luminal site of surgery. It is yet another object of the present invention to ameliorate this problem.
Relevant disclosures of intra-luminal surgical element deployment system are disclosed in Applicant's earlier WO 01/34061 (WO 061) and WO 03/003944 (WO 944) the contents of which is hereby incorporated by this reference. For placement of a stent, the system may include an annular pusher element which abuts the stent to stop it moving proximally when the outer sheath is withdrawn proximally to release the stent.
In one embodiment of the system disclosed in WO 944 there is included a pusher assembly for a delivery system for a self-expanding stent, the pusher assembly constituting a catheter shaft with a proximal pusher end to receive an end-wise compressive force and a distal pusher end to deliver said force to a stent to be delivered, the pusher assembly comprising a pusher strand extending from the proximal pusher end to a distal strand end which is nearer the distal pusher end than the proximal pusher end; a pusher element which abuts the stent in use to deliver said force to the stent; and a transfer shaft having a proximal and a distal end, the proximal end being connected to the distal tube end and the distal end being connected to the pusher element and wherein the pusher element defines a guidewire path, and the transfer shaft lies to one side of said path.
By contrast, in conventional systems such as that of EP 634 in which the atraumatic tip is carried on the inner catheter, the pusher element is mounted on a tube which has a guidewire lumen and extends distally all the way to the tip.
Embodiments of the system of WO 944 provide a stent delivery system having a rapid-exchange configuration for a self-expanding stent which provides improved visualisation through an increased volume flow in each pulse of radiopaque contrast medium pumped through the device. The volume flow in each pulse is increased in the present invention due to a simplified and reduced internal structure of the delivery system.
The fundamental disclosure of Gianturco U.S. Pat. No. 4,580,568 reveals the essential features of a basic delivery system to be an outer sheath confining a stent in a radially compressed state and a pusher element preventing proximal movement of the stent when the outer sheath is proximally withdrawn. The pusher element is carried on an inner catheter shaft. The delivery system is inserted over a guidewire into a lumen of a human or animal body.
A preferred embodiment of the system of WO 944 gets back some way towards the simplicity of such a delivery concept by shortening the inner catheter shaft so that its distal end is relatively close to the proximal guidewire lumen exit port. By contrast, in the by now conventional tip arrangement of a self-expanding stent delivery system the inner catheter shaft extends more distally, even beyond the distal end of the stent, to provide a tapered tip, for ease of insertion of the delivery system into the patient's body and for reducing trauma whenever the catheter is advanced distally. Above-mentioned EPO 634 discloses a stent delivery system which conforms to this conventional model.
In preferred embodiments of the system of Applicant's WO 944, the stent pusher element carries a carrier tube which is used to define at least a short distal guidewire lumen. Further, a system tip taper on the distal end of the outer sheath, renders redundant the need for an atraumatic distal tip on the inner catheter distal of the stent. Therefore, the internal structure of the delivery system is more open, which consequently enhances ease of flow and the volume of contrast fluid that can be ejected from the distal end of the delivery system with each successive pulse imposed from the proximal end of the delivery system. Hence, visualisation is improved.
The manufacturing and assembling steps required to get the delivery system ready for use are minimised due to the simplified internal structure. No longer is there a necessity for keeping the stent at a fixed position on the inner catheter shaft while the outer sheath is fitted over the stent. Also, the risk of advancing the stent too far distally and out of the distal opening of the outer sheath during assembly of the delivery system is minimised, since the outer sheath preferred in embodiments of WO 944 comprises the tapered tip which acts as a distal stopper for the stent during assembly. Also, it is worthwhile to note that there are fewer steps during manufacturing and assembly of the stent delivery system, which itself is a valuable gain in this technical field.
The systems illustrated in Applicant's WO 061 and WO 944 include a self-expanding stent confined within a sleeve that has a heat-formed tapered distal tip. The stent is loaded into the sleeve from the proximal end of the sleeve and, upon deployment of the stent, the tapered tip is drawn proximally over the whole length of the stent to release the stent progressively.
The introduction of a stent using a preferred embodiment of the stent delivery system of WO 944, and subsequent removal of the delivery system, is facilitated especially in tortuous vessels and other body lumens having a relatively narrow diameter because, once the stent has been placed at a desired site inside the patient's body, there need be no component of the delivery system which is radially inwardly located from the stent and which has to be proximally withdrawn through the stent lumen. Especially in narrow and sharply curved body vessels, this might introduce a risk that the distal tip being withdrawn through the stent lumen interferes with bodily tissue protruding radially inwardly through the interstices of the stent and into the stent lumen. A preferred delivery system avoids this problem by providing the tapered tip on the distal end of the outer sheath so that, during removal of the delivery system out of the patient's body, there need be no system components which travel proximally within the stent lumen and are likely to engage with the inner surface of the stent.
In one preferred embodiment revealed in WO 944, the pusher element is a cylinder which has a distal-facing end face at the distal end of the cylinder to push on the proximal end of the stent. The end face is flat and transverse to the axis of the cylinder. The pusher element can serve as, and preferably does serve as, a radiopaque marker.
If desired, the pusher element can also serve as a mount for a distal marker carrier tube cantilevered distally forward from the pusher element to lie within the space that will correspond to the lumen of the stent to be deployed by the system. This is useful when it is required to have on the delivery system a radiopaque marker for the distal end of the stent. This radiopaque marker can be placed on the carrier tube at a position at or towards the distal end of the carrier tube and corresponding to the distal end of the stent. For stents of different lengths, the length of the carrier tube can easily be varied to correspond to the stent length, prior to fixing the distal marker on the carrier tube.
It will be appreciated that the carrier tube requires relatively little strength, so can be made thin and flexible, thereby reducing the risk of its interfering with tissue protruding through the stent during its withdrawal from the stenting site.
As the carrier tube is a relatively simple and isolated part of the delivery system, and conveniently made of a synthetic polymeric material, it will be a relatively simple matter to change the length of the carrier tube to suit any particular stent destined to be carried on the system. If desired, the carrier tube can be extended backwardly proximally from the pusher element and given an end flared outwardly proximally. This flared end provides security against the possibility of unwanted slippage of the carrier tube distally through the pusher element and of being left behind in the body when the delivery system is withdrawn. It may also be useful to guide the guidewire through the system whenever there is need to introduce the distal end of the guidewire from the proximal end of the system.
Another option for modularisation is given by a transfer shaft connecting the rod or inner catheter with the pusher element. This can be set to any desired length, to accommodate stents of different length in a delivery system which features standard length catheter components such as the sheath, rod or inner catheter and pusher tube. It may be convenient to use a welded joint to fasten one or both of the two ends of the transfer shaft to the pusher element and rod, respectively.